Odds are about 50-50 that a sexually active woman will contract the virus that causes cervical cancer at some point in her life, but researchers in Hawaii have discovered that the risk of infection may be just as high in another disturbing site.

Rates of women who are opting for preventive mastectomies, such as Angeline Jolie, have increased by an estimated 50 percent in recent years, experts say. But many doctors are puzzled because the operation doesn't carry a 100 percent guarantee, it's major surgery -- and women have other options, from a once-a-day pill to careful monitoring.

Anal infections of human papillomavirus, known as HPV, appear to be as common as cervical infections, according to an article published in the April issue of the Journal of Infectious Diseases.

Half of the women in the multi-year study acquired new anal HPV infections during the trial period. Counting women infected at the start of the study, roughly 70 percent of women tested positive for anal HPV during clinic visits.

That's similar to known risks for cervical HPV infection, said Brenda Y. Hernandez, a lead author of the study and an assistant professor at the Cancer Research Center of Hawaii in Honolulu.

"What we're seeing here is likely to be comparable to what is seen among primarily young, sexually active women on the U.S. mainland," Hernandez said.

The suggestion that the risk of anal HPV infection is as common the risk of cervical infection is a new one, said Dr. Debbie Saslow, director of breast and gynecologic cancer for the American Cancer Society.

It’s not clear exactly how the women contracted anal HPV. Those who developed infections were more likely to be young and white, with lower levels of education and income and a history of multiple sexual partners, the study showed. Women who engaged in anal sex were also at higher risk, though transmission could have occurred in other, non-sexual, ways.

The analysis was conducted among more than 430 college and community health clinic patients in Hawaii between 1998 and 2003. Although the study did not include a population-based sample, it likely reflects larger social trends, Hernandez said.

The findings are important because anal HPV infection is strongly linked with anal cancer, a rare but increasing disease that famously afflicted 1970s superstar Farrah Fawcett in 2006. The cause of Fawcett's cancer isn't known, according to media reports.

More than 4,600 people are diagnosed with anal cancer in the U.S. each year, and nearly 700 die from the disease. Incidence of the disease has been increasing slightly in recent years, cancer experts said.

However, it's still less than half the nearly 12,000 cervical cancer cases diagnosed in women each year, and a only fraction of the nearly 3,700 deaths.

Although the cause of anal cancer remains unknown, about 90 percent of anal tumors show signs of HPV, researchers have found.

HPV is the leading cause of cervical cancer in women, but it can also cause penile cancer in men and genital warts and other cancers in both sexes.

HPV is actually a group of more than 100 related viruses. Of those, about 30 to 40 percent infect the genitalia and about 15 percent cause cancer. Because the virus causes no symptoms and usually clears up without treatment, many people who have it don’t know it. Recurring infections, however, can boost the risk of cervical and other cancers, experts said.

Clues for sexual behavior
The study results are also important because they reveal additional information about the prevalence of HPV, the most common sexually transmitted infection in the U.S., Saslow said.

“People need to be aware of the risk and make personal decisions about behavior,” she said.

Other recent studies have suggested that sexual behavior such as oral sex may be associated with HPV cancers of the mouth and throat, and the same may be true of anal cancers, though there's not a proven link.

The Hawaii study showed a greater risk of HPV infection in women who recently had anal sex, though the association wasn't as high as researchers expected. Non-penetrative sex and use of fingers and sex toys also may have contributed to transmission of HPV, or the virus could have been shed from cervical secretions, the report said.

“It is also possible that responses to our questions regarding anal sex were less than candid,” the authors wrote.

Study participants were twice as likely to contract the high-risk strains of the HPV virus associated with cervical and other cancers than the low-risk variations of the virus, the report showed.

Because anal cancer is so rare, occurring in fewer than two of every 100,000 people, the higher risk of anal HPV infection shouldn’t cause women undue worry, said Hernandez, who noted that cervical cancer is obviously a higher public health priority.

No one is suggesting adoption of the routine screenings that detect 1 million cases of precancerous conditions in the cervix, said Saslow. “We don’t recommend any anal testing or anal Pap testing,” she said.

New vaccines may protect men and women
Nevertheless, it’s possible that new vaccines that target cervical HPV also will help decrease anal cancer. The only available vaccine, manufactured by Merck & Co., protects against the four strains of HPV that cause most cases of cervical cancer and genital warts.

Finding a way to decrease cases of anal cancer would be a boon, said Dorothy Hendrickson of Streamwood, Ill., who was diagnosed with the disease three years ago at age 58. Doctors say they don't know what caused her cancer, she said.

“You think the cancer is bad? The treatment is horrendous,” said Hendrickson, who works for a freight company. “I spent six months lying on the bathroom floor.”

Her illness preceded Farrah Fawcett’s public diagnosis of anal cancer, and the reported return of the star’s disease after treatment. Hendrickson was among the first fans to post good wishes for Fawcett on a cancer blog.

She said she feels a special kinship with the former "Charlie’s Angel" star because the two were born just days apart in February 1947, and because they’ve both battled the rare disease.

“I felt so bad because I knew what she was going through,” Hendrickson said.